1. Technical Field of the Invention
The present invention relates to a rotation balance correcting method of a motor-driven supercharger provided with an electric motor assisting a rotational drive of a supercharger rotor, and a rotation balance testing apparatus thereof.
2. Description of the Related Art
In order to improve a performance of an internal combustion engine, there has been widely used a supercharger (also called as “turbocharger”) driven by an exhaust gas of the internal combustion engine so as to compress an intake air and supercharge. Further, there has been known a supercharger in which an acceleration response or the like is improved by incorporating an electric motor on the same axis as a shaft of a supercharger, and accelerating and assisting a rotational drive of a compressor. The supercharger having a motor-driven assist function by the electric motor is called a “motor-driven supercharger.”
FIG. 4 is a cross sectional view showing a structure of a general motor-driven supercharger 10.
A compressor impeller 6 and a compressor housing 8 surrounding it are arranged in an air supply passage side of the motor-driven supercharger 10.
A turbine impeller 2 and a turbine housing 4 surrounding it are arranged in an exhaust passage side of the motor-driven supercharger 10.
The turbine impeller 2 is integrated with a shaft 12 by welding or the like, and the shaft 12 is fitted to a center portion of the compressor impeller 6. The compressor impeller 6 is fastened by a nut 7.
The shaft 12 is rotatably supported by a journal bearing 16 and a thrust bearing 17 built in a center housing 14.
On the basis of this structure, the turbine impeller 2, the compressor impeller 6 and the shaft 12 are integrally rotated. Hereinafter, a rotating body constituted by the turbine impeller 2, the compressor impeller 6 and the shaft 12 is called as “supercharger rotor”.
Further, the center housing 14 has an electric motor 18 having a rotor 18A coaxially coupled to the shaft 12, and a stator 18B arranged around the rotor 18A built-in.
In the motor-driven supercharger 10 structured as mentioned above, the turbine impeller 2 is rotated by an exhaust gas from the internal combustion engine (the engine), the compressor impeller 6 coupled thereto is rotationally driven and the rotational driving is assisted by the electric motor 18, and an intake air is supercharged so as to be supplied to the internal combustion engine.
On the other hand, in the motor-driven supercharger 10, a rotation balance test is executed for every product before shipping the product so as to measure an unbalance amount, and a rotation balance correction is executed. Particularly, in the motor-driven supercharger 10, the shaft 12 becomes longer at a degree that the electric motor 18 is attached, in comparison with the supercharger in which the electric motor is not mounted, and a high-level balance correction is required.
The conventional rotation balance correcting method of the supercharger is disclosed, for example, in the following patent document 1. FIG. 1 is a view explaining the rotation balance correcting method in the patent document 1, and this method is constituted by the following steps (A) to (E).
(A) Attaching a turbine housing 51 of the supercharger 50 in a state of detaching only a compressor housing to a turbine housing mounting plate 53 of a vibration table 52.
(B) Attaching an accelerator pickup 54 to a flat surface 53a of the turbine housing mounting plate 53.
(C) Arranging a rotation detector 55 near a compressor impeller 56.
(D) Supplying a compressed air to a turbine impeller 57 and rotating the turbine impeller 57 at a high speed.
(E) If reaching a predetermined high-speed rotating speed, detecting an acceleration (a vibration) of the supercharger 50 by the accelerator pickup 54, detecting an angle of rotation of the compressor wheel 56 by the rotation detector 55, and correcting a high-speed unbalance amount of the supercharger rotor on the basis of a result of computation of the detected input signals in a computing element 58.
Further, the following patent document 2 discloses striking a rotation balance as an entire supercharger rotor while detecting a vibration by a vibration pickup provided in a supercharger housing, by driving a turbine rotor (a turbine impeller) by a compressed air having a high temperature equal to or more than 400° C.    Patent Document 1: Japanese Unexamined Patent Publication No. 2002-39904    Patent Document 2: Japanese Unexamined Patent Publication No. 2004-116317
In the prior arts disclosed in the patent documents 1 and 2 mentioned above, the rotation balance correction is executed by driving the turbine impeller by the compressed air, rotating the supercharger rotor at a high speed, and detecting the data necessary for correcting the rotation balance. As mentioned above, since a compressed air source is necessary for rotating the supercharger rotor at a high speed, a rotation balance testing apparatus is enlarged in size.
Furthermore, the supercharger is generally designed so that a pressure balance in an inner portion of the supercharger is correctly adjusted; however, in the case that the supercharger is mounted on the engine, in accordance with the prior art disclosures mentioned above, because the rotation balance test is executed in the state in which only the compressor housing is detached, the normal pressure balance is not secured. In other words, explaining with reference to FIG. 4, if only the compressor housing 8 is detached, and the compressed air is supplied to the turbine impeller 2, a force in a left direction in FIG. 4 is applied to the turbine impeller 2 on the basis of the pressure balance in a periphery thereof. Accordingly, an excessive axial load (a counterthrust force) in the turbine side is applied to the thrust bearing 17 via the shaft 12, and the thrust bearing 17 is adversely affected.
Further, since the turbine impeller 2 is covered by the turbine housing 4, it is necessary to detach the turbine housing 4 and a piping connected thereto in the case that the balance correction is necessary in the turbine impeller 2 side, so that the balance correcting work is complicated.